1. Field of the Invention
The invention relates to catalytic reduction of oxides of nitrogen, NO.sub.x, produced in the bubbling dense bed regenerators associated with catalytic cracking unit regenerators operating in complete CO combustion mode.
2. Description of the Related Art
NO.sub.x, or oxides of nitrogen, in flue gas streams from FCC regenerators operating in complete CO burn mode is a pervasive problem. FCC units process heavy feeds containing nitrogen compounds, and much of this material is eventually converted into NO.sub.x emissions. There may be some nitrogen fixation, or conversion of nitrogen in regenerator air to NO.sub.x, but most of the NO.sub.x in the regenerator flue gas is believed to come from oxidation of nitrogen compounds in the feed.
Although all FCC regenerators produce some NO.sub.x, the problem is more severe in bubbling bed regenerators, as opposed to high efficiency regenerators. High efficiency regenerators burn most of the coke in a fast fluidized bed coke combustor. Such regenerators have few stagnant regions. Bubbling bed regenerators may have stagnant regions and will have large bubbles of air passing through the bed, leading to localized areas of high oxygen concentration. Although the reasons for the different NO.sub.x emissions in these two type of regenerator are perhaps not completely understood, all agree that NO.sub.x emissions are usually significantly higher, frequently twice as high, from bubbling bed regenerators.
Several powerful ways have been developed to deal with the problem. The approaches fall into roughly five categories:
1. Feed hydrotreating, to keep NO.sub.x precursors from the FCC unit. PA1 2. Segregated cracking of fresh feed. PA1 3. Process approaches which reduce the amount of NO.sub.x formed in a regenerator via regenerator modifications. 4. Catalytic approaches, using a catalyst or additive which is compatible with the FCC reactor, which suppress NO.sub.x formation or catalyze its reduction. 5. Stack gas cleanup methods downstream of the FCC unit.
The FCC process will be briefly reviewed, followed by a review of the state of the art in reducing NO.sub.x emissions.